Generally, in a base station device (CDMA base station device) having a mobile communication system which adopts W-CDMA (Wideband Code Division Multiple Access) scheme as a mobile communication scheme, it is necessary to cause a radio signal to reach a remote mobile station device (CDMA mobile station device). Thus, there is need for transmitting and outputting a signal to be transmitted having been highly amplified by an amplifier.
The amplifier, which is an analog device, exhibits nonlinear input/output characteristics. In particular, the output power level of the amplifier becomes almost constant even when the input power level of the amplifier increases beyond an amplification limit referred to as a saturation point. Such nonlinear output leads to a nonlinear distortion in the output signal.
Typically, signal components outside of a desired signal band are suppressed to a low level by a band-pass filter before a transmission signal is amplified. However, after the transmission signal is amplified at the amplifier, the amplified signal exhibits a nonlinear distortion, entailing the leakage of signal components of the amplified signal into undesired signal bands (adjacent channels).
For example, since the transmission power level at the base station can be high as described above, the level of the leakage power into the adjacent channels need be suppressed below a certain strictly defined level. To this end, techniques for reducing adjacent channel leakage power (ACP) have been used.
As one of the techniques, a technique in which a peak limiter for limiting the maximum power (peak) of the signal to be transmitted to output the maximum power limited signal is introduced at a front end of the amplifier and the maximum power (peak) limited signal is used as an input signal of the amplifier.
As a CDMA base station transmitter having the peak limiter, in Japanese Patent Laid-Open No. 2002-44054 on Feb. 8, 2002 (Applicant: HITACHI INTERNATIONAL ELECTRIC INCORPORATED, and Inventor: SASAKI KOHEI), ‘Combination Carrier Transmission Circuit with Limiter Circuit’ has been suggested.
In the combination carrier transmission circuit with limiter circuit, when a multicarrier is transmitted from the base station, the ratio of instantaneous power to mean power is calculated as an instantaneous peak factor, based on a signal obtained by multiplexing all of the carriers with the limiter circuit (peak limiter). The instantaneous peak factor is compared to a peak factor threshold value which is a reference value. Based on the comparison result, a limit factor suitable for the extent that clipping is needed is outputted. Then, by multiplying the limit factor for each carrier, the peak is limited. Thus, the dynamic range of the amplifier which amplifies the multicarrier can be effectively utilized and the bit error rate in the mobile station can be reduced without unnecessarily limiting the peak. (Patent Document 1)
As Patent Document 1, Japanese Patent Laid-Open No. 2002-44054 (P. 5 to 7 and FIG. 1) is exemplified.
The conventional peak limiter suppresses the maximum power of the input signal to the amplifier. Thus, for the input signal, the mean power of the input signal and the instantaneous power of the input signal are detected. From information of the mean power and information of the instantaneous power, it is detected whether there exists a peak to be power-limited, so that peak detection information is outputted. Then, according to the peak detection information, the peak to be power-limited is detected. In this case, the power level of the input signal may be limited to predetermined limit power level and the output signal limited to the limit power level may be outputted. As a result, the signal limited to the limit power level may be inputted to the amplifier.
Further, in a transmitter which handles multiple carriers, a multicarrier signal in which the multiple carriers are multiplexed (combined) is inputted to the amplifier. Thus, the peak limiter calculates the instantaneous power and the mean power after the carriers are multiplexed and, based on these values, performs the detection of whether there exists the peak to be power-limited.
Here, as a detection method of whether there exists the peak to be power-limited, a method in which the ratio between the instantaneous power and the mean power of the input signal is calculated and, when it is larger than the predetermined peak factor threshold value, judges the peak of the input signal as the peak to be power-limited can be considered. Here, the peak factor is the ratio between the maximum power and the mean power of the input signal of the amplifier as shown in FIG. 8. That is, the smaller the difference of the maximum power from the mean power is, the smaller the peak factor is. FIG. 8 is an explanatory view of the peak factor of a general amplifier.
Typically, the input signal input to the peak limiter is a baseband signal before the band limitation and, after the limiter process is performed by the peak limiter, the band is limited by a filter. Thus, the distortion in the amplifier does not occur. Further, since the peak value of the input signal is limited by the peak limiter, the peak factor of the input signal becomes small. Therefore, the operating point of the amplifier that performs amplification after the band limitation can be raised, which results in enhancing power efficiency.
Here, since the band is limited after the limiter is performed by the peak limiter, the peak factor after the band limitation typically is larger than the peak factor before the band limitation. This is because a rectangular wave before the band limitation becomes obtuse after the band limitation and a point at which the peak becomes high is present. Here, the predetermined peak factor threshold value of the peak limiter needs to be set low in consideration that the peak factor after the band limitation becomes large.
However, in a general CDMA base station transmitter, there is a problem, when the peak limitation (suppression) is performed for the multiple carriers, if the input level of the carrier varies, the level after the peak limitation varies.
As described above, when the peak limitation of the multiple carriers is performed, the peak limiter calculates the instantaneous power and the mean power based on the input level after multiplexing each carrier, that is, the sum of the power levels of the carriers and compares the ratio between the instantaneous power and the mean power to the peak factor threshold value to detect whether or not there exist the peak, and performs the peak limitation for each carrier.
In the peak limiter, the peak factor threshold value is optimized when the respective carriers have the maximum output and the same level. However, when all carriers do not have the maximum output and the same level, the peak suppression is performed. Further, in general, the peak factor threshold value is determined depending on the number of the carriers.
That is, the peak limitation in a peak power suppressing section (peak limiter) of a general transmitter performs the peak power suppression equally for each carrier at a uniform suppression ratio when the peak is detected based on the sum of the power levels of the carriers, on the assumption that, for the device configuration, the carrier signals exist in all of the multi input carriers and the respective carrier signals have the same level. Accordingly, desired peak suppression is achieved with the sum of the power levels of all of the multiple carriers. As a result, the peak factor for the multicarrier becomes small.
When such a peak power suppressing section is used for the transmitter, if the input level of an identified carrier temporarily varies abruptly, the peak limitation (suppression) is performed for all carriers, so that the level after the peak limitation varies. As a result, there is a problem in that the level of the multicarrier signal after the combination varies.
Here, when the input level of any carrier varies while the peak factor threshold value is fixed, the sum of the power levels of the carriers varies. Thus, the peak limitation by the peak limiter is not optimized and the power deviation occurs. That is, the level of the multicarrier signal varies within the range of ±0.3 dB.
Further, when the input level of the identified carrier is constant, if the input levels of all other carriers vary, the sum of the power levels of the carriers also varies. Accordingly, the peak limitation by the peak limiter is not optimized and the power deviation occurs. That is, the level of the carrier having the constant input level varies after the limitation. The power deviation depends on the number of the carriers. When the total number of the carriers is two, the level of the carrier varies within the range of ±0.3 dB, and when the total number of the carriers is four, the level of the carrier varies within the range of ±1.2 dB.
That is, in the case of the general transmitter, when the input level of the identified carrier abruptly varies, the sum of the power levels of the carriers also abruptly varies. Accordingly, even when the mean power does not vary significantly, the instantaneous power becomes abruptly large and the peak factor becomes large, so that the peak is detected. As a result, uniform peak suppression is performed for all carriers and a carrier the input level of which does not vary is greatly influenced by the peak suppression. Therefore, there occurs a first problem in that power is totally lowered and the level of the multicarrier signal is lowered.
Further, for example, when any carrier among the multiple carriers is interrupted, the input level of the identified carrier among operating carriers abruptly varies. Further, when the peak factor of the peak limiter becomes large and the peak is detected, the peak suppression is uniformly performed at the suppression rate on an assumption that all of the carriers including the interrupted carrier have the same level. Accordingly, when any carrier is interrupted, the suppression rate is not sufficient in the peak limiter. Therefore, there occurs a second problem in that the level of the multicarrier signal cannot be sufficiently lowered.
Further, in the peak power suppressing section, as described above, the peak factor threshold value is set low in consideration of a peak which is caused by subsequent band limitation (waveform shaping filter). Thus, there occurs a third problem in that the peak suppression unnecessarily operates in the circuit at the present state.
On the other hand, in the W-CDMA communication scheme, the power control is realized such that the output difference in the transmitter of the base station is equivalent to 0.1 dB or 0.5 dB. However, in the above-described transmitter, the number of the transmission carriers and the input level of each carrier increase or decrease due to the above-described problems. Accordingly, the peak limitation is not sufficiently performed or the peak limitation is excessively performed so that the level is excessively suppressed. As a result, there is a problem in that the output difference according to the rules cannot be achieved.
The present invention has been made in consideration of the above-described problems, and it is an object of the present invention to provide a transmitter that can averagedly suppress a variation in input level of a multicarrier signal to an amplifier according to an increase or decrease in the number of the transmission carriers or a variation in input level of each carrier.